You know, I really think it all depends on your personal definition of "flirting".
Is "flirting" licking your lips, rubbing another person's arm/leg? If so, then it's a problem for anybody in a relationship and shows there are other issues in that relationship.
Is "flirting" smiling at somebody of the opposite sex, holding a conversation with them, joking around? If so, then I don't see the problem with it.
I think casual conversations among people get blown way out of proportion by people who are jealous/insecure about their relationships. :shrug:
I found this article online:
Flirting tips,online flirting,signs of flirting,flirting guide at FlirtingTipsQA
"Flirting is often used as a means of expressing interest and gauging the other person's interest in courtship, which can continue into long term relationships. Alternatively, it may simply be a prelude to casual sex with no continuing relationship.
In other situations, it may be done simply for immediate entertainment, with no intention of developing any further relationship. This type of flirting sometimes faces disapproval from others, either because it can be misinterpreted as more serious, or it may be viewed as "cheating" if the person is already in a romantic relationship with someone else. "
And this article too:
SIRC Guide to flirting
"Flirting is much more than just a bit of fun: it is a universal and essential aspect of human interaction. Anthropological research shows that flirting is to be found, in some form, in all cultures and societies around the world.
...
At one level, you can flirt with more or less anyone. An exchange of admiring glances or a bit of light-hearted flirtatious banter can brighten the day, raise self-esteem and strengthen social bonds. Flirtation at this level is harmless fun, and only the stuffiest killjoys could possibly have any objections. "